Fix cherry-pick job for unmerged PRs

[jakebailey]

When running this task on an unmerged PR, it will be using the merge commit that GitHub generates for every PR. You can't cherry-pick a merge commit without telling it which side of the merge is the intended base. So, the job fails with:

/usr/bin/git config user.email typescriptbot@microsoft.com
/usr/bin/git config user.name TypeScript Bot
/usr/bin/git switch --detach origin/release-5.3
HEAD is now at b4fe221e65 Cherry-pick #56489 into release-5.3 (#56490)
/usr/bin/git switch -c cherry-pick/56504/release-5.3
Switched to a new branch 'cherry-pick/56504/release-5.3'
/usr/bin/git cherry-pick dbd9bbb928adbe9e21cbbd8947729e206e463bcc
error: commit dbd9bbb928adbe9e21cbbd8947729e206e463bcc is a merge but no -m option was given.
fatal: cherry-pick failed
Error: The process '/usr/bin/git' failed with exit code 128

By specifying -m 1, we can tell git that HEAD^1 is the merge base (i.e. main), and the cherry-pick will work.